X-Ray Ionization of the Disks of Young Stellar Objects

We have developed a Monte Carlo code for the transport of stellar X-rays in an axially symmetric disk. The code treats Compton scattering and photoelectric absorption and follows the X-rays until they are completely absorbed. We confirm that hard X-rays from a low-mass young stellar object (YSO) penetrate the associated accretion disk. Even without the low-energy photons that are strongly attenuated by the YSO wind, the ionization rate in the inner region of the accretion disk (<1 AU) is many orders of magnitude larger than the standard ionization rate due to Galactic cosmic rays. At a fixed radius from the source, the X-ray ionization rate is a universal function of the vertical column density, independent of the structural details of the disk. The ionization rate scales with the X-ray luminosity and depends only mildly on the X-ray temperature, at least for the temperatures relevant for low-mass YSOs. Thus X-rays from a YSO can ionize regions of an accretion disk from which low-energy cosmic rays are excluded, e.g., by the action of stellar winds. Using a simple theory for the electron fraction we estimate that, for a minimum solar nebula, X-rays ionize the disk beyond 5 AU at a level sufficient to couple magnetic fields and neutral disk material. Inside this radius, the X-rays are ineffective for vertical column densities much larger than ~10²⁵ cm^-2, and thus an interior region of the disk will be uncoupled from magnetic fields. If disk accretion is mediated by MHD turbulence, as proposed by Balbus & Hawley, then our results suggest that layered accretion occurs in the inner regions of a disk ionized by X-rays, in accord with Gammie's suggestion based on cosmic-ray ionization.